1. Field of the Invention
This invention relates to a novel interferometric apparatus for accurately and nonintrusively measuring surface skin friction on a model or a body in an air flow and for accurately measuring the viscosity of the oil used for the skin friction measurement.
2. Description of the Prior Art
There are numerous devices in common usage to measure skin friction. The commonest are Preston tubes, surface thin-film heat-transfer gages and floating-element balances which are used to make mean-velocity profile boundary layer surveys for use in conjunction with the Clauser chart. These devices are described and discussed in detail in "An Outline of the Techniques for the Measurement of Skin Friction in Turbulent Boundary Layers," K. G. Winter, Progress in the Aerospace Sciences, Vol. 18, Pergamon Press, Great Britain 1977, pp. 1-57. Except for the floating-element balance, all of the aforementioned devices measure skin friction indirectly since they are based on the wall similarity in turbulent boundary layers. The floating-element balance does measure skin friction directly; however, it has critical gap and alignment problems, especially when subjected to a pressure gradient. Further, these balances are very delicate and expensive. All of the mentioned instruments are undesirable in that they must be installed in the surface being measured or they must intrude in the flow.
Tanner has described a skin friction instrument that overcomes some of the limitations of the previously noted devices. "A Study of the Motion of Oil Films on Surfaces in Air Flow, With Application to the Measurement of Skin Friction," L. H. Tanner and L. G. Blows, Journal of Physics E: Scientific Instruments, Vol. 9, 1976, pp. 194-202; "A Skin Friction Meter, Using the Viscosity Balance Principle, Suitable for Use with Flat or Curved Metal Surfaces," L. H. Tanner, Journal of Physics E: Scientific Instruments, Vol. 10, 1977, pp. 278-284; "A Comparison of the Viscosity Balance and Preston Tube Methods of Skin Friction Measurement," L. H. Tanner, Journal of Physics E: Scientific Instruments, Vol. 10, 1979, pp. 627-632. The instrument uses a laser to interferometrically measure the thickness of an oil film flowing on a surface subject to skin friction. The thickness measurements are, in turn, used with a theory that describes the flow of the oil to compute ski friction. Although it is a direct method and does not require calibration in a known flow, its use has not been widely adopted for measuring skin friction because of several shortcomings. Tanner employs a first layer beam to measure oil thickness. Tanner's oil-flow theory requires a very accurate determination of the distance between the oil film leading edge and the laser beam focal point. Tanner employs a second laser beam (with fixed spacing from the first beam) as a "spotter beam" and visually positions the beam at the leading edge of the oil before a test. This manual distance measurement method is inconvenient at best and may be impossible in many wind tunnels because of visibility limitations. The measurement is also prone to error because of the subjective nature of visually locating the oil leading edge, and because the oil applied to the surface is not confined, the leading edge can significantly move between the time the spotter beam is positioned on the oil leading edge and the time the wind tunnel is started. [When Tanner encountered problems caused by wind tunnel vibrations in the two-beam measurement instrument, he tried an alternate apparatus and that also failed to obviate the aforementioned problems. The second embodiment utilized a single laser beam. Tanner initially visually aimed the beam at the oil leading edge and then moved the beam downstream with a slip gauge. A micrometer was used to measure the spot displacement.] Tanner's skin friction process requires that the total oil flow time be accurately known and he provides no way of measuring it. Tanner merely assumes that the oil flow commences when the wind tunnel is started. This assumption is subject to unknown error because of prerun oil flow, tunnel starting transients, and initial surface waves which form on the oil. These limitations have the cumulative effect of making Tanner's skin friction measurement method impractical for wind tunnel testing.